1. Field of the Invention
The present invention relates to an image display apparatus. More specifically, the present invention relates to an organic electroluminescence display using an organic electroluminescence device.
2. Related Background Art
An organic electroluminescence (hereinafter referred to as EL) device utilizing the EL of an organic material is constituted by interposing an organic compound layer, which is obtained by laminating a light emission layer and a carrier transport layer each composed of an organic molecule, between a lower electrode and an upper electrode. A display using the organic EL device is particularly suitable for displaying a color dynamic image because the display is excellent in color reproducibility, and has good response to an input signal. In addition, the display can be used in a wide variety of environments because the display can emit light at high luminance, and has a wide view angle.
Examples of a material for the organic compound layer include a low-molecular weight material that can be subjected to vacuum deposition and a polymer-based material that qualifies for application by means of a spin coating method or an ink jet method. Although a low-molecular weight material has been used in many cases at present, the number of cases where a polymer-based material suitable for a large-area display is used is expected to increase in the future. Methods of driving the device are classified into a simple matrix type and an active matrix type. In the simple matrix type, the device is composed only of stripe-shaped lower and upper electrodes extending in the directions perpendicular to each other. In the active matrix type, each pixel has a thin film transistor for driving the organic EL device.
An organic EL display has, for example, the following characteristics: a more beautiful screen, faster response, and relatively lower power consumption than those of a liquid crystal display or of a plasma display. At present, however, the organic EL display has been put to practical use only in applications where the display is used in a small screen and is continuously used for a relatively short time period such as: the indicators of various measuring instruments; and the display screens of a portable phone and a digital camera. The main problem to be solved before the organic EL display is used in applications where the display is used in a large screen and is continuously used for a long time period such as the monitors of a television and a computer in the future is a reduction in light emission intensity in association with the long-term, continuous use of the display.
A reduction in light emission intensity often starts from the peripheral portion of a pixel or in a spot fashion, and measures provided by a drive circuit according to a voltage programming mode or current programming mode to be described later do not sufficiently alleviate the reduction. At present, a cause for the phenomenon has not been completely ascertained. However, an organic compound to be used in an EL display is extremely sensitive to moisture. Accordingly, there is no doubt that the reaction of the organic compound itself or of an interface between an organic compound layer and an electrode caused by so trace an amount of moisture that cannot be analyzed is greatly responsible for the deterioration of the properties of the display.
In view of the foregoing, the surface of an organic EL device is generally provided with a protective layer so that the penetration of moisture from the outside of the device is inhibited. It is particularly recommended that a film having a coefficient of water permeability of 10−5 mg/m2·day or less made of, for example, silicon nitride or polyparaxylene be used. Furthermore, the outermost surface of the device is covered with a cover glass. In addition, even a trace amount of penetrated moisture is adapted to be absorbed by a drying agent such as calcium oxide, calcium chloride, or barium oxide, so the lifetime of an organic EL display is effectively lengthened. Despite such measures as mentioned above, the lifetime of a current organic EL display does not reach the level needed for application to a television, that is, 50,000 hours. Not only an influence of moisture penetrating from the outside of an organic EL display during the use of the display but also an influence of moisture that is absorbed by a film or generated during a production step for the display must be investigated with a view to achieving an additional increase in lifetime of the display.
In a production step for an organic EL display, a planarization film or a device separation film frequently contacts with a developer or cleaning fluid containing moisture before an organic compound layer is formed. Accordingly, each of the films may absorb moisture in it, or may adsorb moisture to its surface. In addition, as described later, a polyimide film may produce moisture in association with a formation reaction for the film. Possible measures to cope with such circumstances include heating and a dehydration treatment in an atmosphere with reduced pressure. However, a heating treatment among others is preferably performed before the formation of the organic compound layer because a material for the organic compound layer decomposes at a temperature in excess of 100° C.
Japanese Patent No. 3531597 describes that moisture incorporated into or adsorbed to an insulating layer (corresponding to a planarization film or a device separation film) constituting an organic EL device diffuses to the entirety of the device in association with the long-term use of the device to cause a reduction in light emission intensity particularly at the peripheral portion of a pixel. The document proposes that the insulating layer is subjected to a dehydration treatment by heating a substrate at 80° C. or higher before the formation of an organic compound layer (thin film layer) or by placing the substrate in an atmosphere with reduced pressure for 10 minutes or longer. The document describes that no significant change in light emission efficiency is observed even after 1 to 3 months from the production of the device owing to the improvement.
However, the inventor of the present invention has found that, in the case where an organic EL display is continuously used over 1 to 3 months, or is stored at a high temperature as about 85° C., a reduction in light emission intensity similar to that in the case of a conventional organic EL display described in Japanese Patent No. 3531597 is observed even when the measures disclosed in Japanese Patent No. 3531597 are taken. Therefore, only the method of Japanese Patent No. 3531597 may not suffice for the dehydration of the insulating layer. When the substrate immediately before the formation of the thin film layer is heated up to a temperature much higher than 80° C. in an atmosphere with reduced pressure, a dehydration effect becomes relatively high. However, the substrate must be prevented from being heated up to a temperature exceeding a certain limit because a large amount of low-molecular weight components may volatilize from the insulating layer in association with the heating, or a strong stress may be generated owing to thermal contraction. In particular, in the case of an active matrix type device, a transistor or a capacitor is present below a planarization film, so excessive heating adversely affects the properties of the device. Heating at about 80° C. for a long time period is not preferable in terms of productivity because the heating causes a reduction in throughput although the heating is somewhat effective.
In view of the foregoing, the inventor of the present invention has conducted detailed investigation into a formation step for a polyimide. It is said that a polyimide has high heat resistance, is stable and absorbs or permits the permeation of a small amount of moisture under appropriate forming conditions for the polyimide; and shows particularly excellent properties when it is used as a device separation film or a planarization film.
An example of the formation of a polyimide film is a reaction involving: synthesizing a polyamic acid that is soluble in an organic solvent by using pyromellitic anhydride (PMDA) and bis(4-aminophenyl)ether (ODA) as starting materials; and imidating the polyamic acid as a precursor through heating to form a polyimide film. The reaction is shown below (Japan Polyimide Research Group, “Latest Polyimide—Basis and Application—”, published by NTS, p. 4-5). That is, the polyamic acid dissolved into an organic solvent is applied, and the organic solvent is driven off in a pre-baking step, whereby the film is cured. The polyamic acid shown here has no photosensitivity, so the formation of an opening requires the separate application of a resist and the performance of exposure and development. After the formation of the opening, washing is performed for removing a developer and a residue. Furthermore, post-baking is performed, whereby an amino group and a carboxyl group cause ring closure to form an imide group. Water is produced as a by-product in association with the ring closure.

An example of the reaction of a photosensitive polyimide capable of simplifying a formation step for a polyimide film is shown below. In this case, a methacryloyl group having photosensitivity as represented by R in the formula is bound to a carboxyl group of a polyamic acid via an ester, so the polyamic acid becomes insoluble in a developer when it is irradiated with light. Therefore, an unexposed portion is removed after development, whereby an opening is formed. After that, as in the case of the above-described non-photosensitive material, washing and post-baking are performed, whereby an amino group and a carboxyl group cause ring closure to form an imide group (Japan Polyimide Research Group, “Latest Polyimide—Basis and Application—”, published by NTS, p. 4-5). In this reaction, alcohol is formed in association with the ring closure. The photosensitive material is of a negative type in which an unexposed portion is removed by development. In contrast, a positive type photosensitive material in which an exposed portion is removed by development is also available (Japanese Patent Application No. 2004-334089), and has been widely adopted in the production of an organic EL display because a taper can be easily formed at an opening portion of a device separation film.

Two concerns can be pointed out here. A first concern is that such non-photosensitive material as represented by Chemical Formula 1 forms water in association with the ring closure of an imide group. A second concern is that a film in an unstable state before the formation of the imide group is quite likely to absorb moisture irrespective of the presence or absence of photosensitivity because the film is caused to contact with a developer or a cleaning fluid in an opening forming step or a washing step for forming an opening. Moisture absorbed by the film is expected to be released by high temperatures in the subsequent post-baking step. On the other hand, the moisture permeability of the film reduces as soon as imidation progresses, so moisture in the film is hardly released. Therefore, the release of moisture in a film may not be considerably expected from the dehydration treatment after post-baking described in Japanese Patent No. 3531597 although the treatment is effective in removing moisture adsorbed to the surface of the film.
As described above, minimizing the amount of moisture finally remaining in a planarization film or in a device separation film is extremely important to put an organic EL display to practical use. However, it is difficult to quantitatively analyze an extremely trace amount of moisture that may affect the properties of an organic EL device.
In view of the foregoing, the inventor of the present invention has made attempts to indirectly determine as to what kind of state of a polyimide film has a small amount of remaining moisture. Japanese Patent Application No. 2004-111361 describes a reference for determining. That is, Japanese Patent Application No. 2004-111361 focuses on an imidation ratio as a criterion for determination as to whether a formed polyimide film is good or bad when the polyimide film is used as an insulating layer of an organic EL device, and describes that an imidation ratio in excess of 95% is preferable. However, the document has no description concerning how the imidation ratio is related to the lifetime of the organic EL device or to the amount of moisture remaining in the insulating layer. Furthermore, the document has no description concerning how the film is evaluated for imidation ratio, so the document provides no specific clue to determine.